In systems which utilize higher frequency electromagnetic radiations such as X rays, gamma radiation or the like, it is often necessary to directionalize a radiation flux that is initially composed of rays traveling in diverse different directions. This cannot be accomplished with refractive lenses, reflective mirrors or the like as in the case of the optical band of frequencies. Instead, it is necessary to employ a collimator which is basically a body of radiation absorbent material transpierced by one or more radiation transmissive passages. When placed between the radiation source and the device or subject to which radiation is to be transmitted, the collimator absorbs intercepted radiation other than intercepted radiation which is traveling along paths coincident with the passage or passages of the collimator.
The operation of certain forms of radiation utilizing system may be enhanced by employing collimators having structural characteristics that are difficult to realize by using known construction techniques, such as by simply drilling the desired passages through a block or plate of radiation absorbent material. One example of such a system is described in prior U.S. Pat. No. 3,949,229, issued Apr. 6, 1976, to the present applicant and entitled, X-RAY SCANNING METHOD AND APPARATUS.
The above identified prior patent discloses a radiographic system for producing an instantaneous X-ray image of a subject on the screen of a display device such as a television receiver set. An X-ray source at one side of the subject generates a moving X-ray origin point which is swept along successive scan lines of a raster pattern area on a broad anode plate. At least one radiation detector, of very small size in relation to the raster area of the source, is situated at the other side of the region to be imaged. The raster sweep frquencies of the display device are synchronized with those of the X-ray source and the electron beam intensity of the display device is modulated by the output of the radiation detector to generate the radiographic image on the screen of the display.
The above described system of prior U.S. Pat. No. 3,949,229 preferably employs a broad multiple apertured collimator situated between the X-ray source and the subject. To be most effective, the collimator should have an extremely large number of very small and closely spaced radiation passages which in some cases should be convergent so that each passage is directed toward the small radiation detector at the other side of the subject. A collimator with such characteristics has several beneficial effects in a radiographic system of the kind described above. Radiation dosage of the subject, which may be a medical or dental patient, is greatly reduced since the collimator suppresses radiation from the source that is traveling in the general direction of the subject but which is not directed at the small detector and which therefore could not contribute useful information to the image. The collimator also enhances image clarity by reducing secondary X-ray production at random origin points within the subject. Such secondary X rays can otherwise introduce spurious data into the image.
Similar collimators having a very large number of minute radiation passages are useful in a variety of other radiation systems, another example of such a system being described in prior U.S. Pat. No. 4,144,457 entitled, TOMOGRAPHIC X-RAY SCANNING SYSTEM, issued Mar. 13, 1979, to the present applicant.
Structural characteristics of the collimator have a pronounced effect on the performance of X-ray systems of the kind discussed above. Definition and clarity of the image is in part a function of the number of radiation passages which can be provided per unit area of the collimator. Providing of a greater number of passages per unit area in turn dictates that passage size be reduced. For example in some systems, it would be desirable to provide as many as one hundred passages per linear centimeter of collimator surface with cross-sectional passage dimensions of the order of 25 microns. As a practical matter, prior collimator construction methods are incapable of realizing such parameters.
In many circumstances the performance of such a collimator is also dependent on maximizing transmissivity which is the ratio of intercepted radiation, which is traveling in the desired directions, that is transmitted through the collimator as opposed to being absorbed. Maximizing transmissivity is dependent on the degree to which the spacing between the radiation passages of the collimator can be minimized. It is also in part dependent on the cross-sectional configuration of the passages. Circular cross-sectioned passages, such as produced by conventional drilling methods for example do not maximize transmissivity. Passages of polygonal cross section would be more effective for this purpose. The difficulties of producing passages of noncircular cross-sectional configuration by known techniques greatly increases if the passages are to be of minute cross-sectional area as discussed above.
Using prior collimator constructions and fabrication techniques it is also very difficult to control the alignments of many small passages with the desirable degree of precision and again this problem is aggravated to the extent that the number of passages per unit area is increased and the size of each individual passage is reduced.
For optimum collimator performance, each individual passage should establish a radiation path having a precise predetermined orientation relative to the paths established by each of the other passages. Achieving this precision can be difficult in the manufacture of collimators in which the passages are intended to be parallel and the problems are still more pronounced in the manufacture of focusing collimators. Focusing collimators have passages which are convergent towards a single distant focal point. Thus in collimators of this particular kind no two of the extremely large number of minute passages have exactly the same orientation in the collimator but the differences in the orientation of adjacent passages may be very slight. If a series of focusing collimators, each having a different focal length, are to be manufactured, the problems of obtaining precision in the orientation of the passages are compounded.
Conventional collimator constructions often also result in an undesirably costly product in that more of the radiation absorbent material is present in the collimator, to provide structural integrity, than is actually needed strictly from the standpoint of performing the collimating function. Such materials are typically heavy metals some of which are relatively costly. A related factor is that the inclusion of more heavy radiation absorbent material than is actually needed to achieve the collimating function increases the weight of the collimator. In some systems, such as certain forms of the apparatus disclosed in applicant's copending application, U.S. Pat. No. 4,259,583, filed concurrently herewith and entitled, IMAGE REGION SELECTOR FOR A SCANNING X-RAY SYSTEM, it is preferable that the weight of the collimator be minimized.
While the problems encountered with prior collimator constructions and methods of manufacture have been discussed above primarily with reference to scanning X-ray systems, similar collimator problems are also encountered in other apparatus. For example in more conventional radiographic procedures for medical or dental purposes or the like X rays are produced at a fixed origin point in an X-ray tube and travel, through the region of the patient to be examined, to a relatively broad film or fluorescent screen. Image degradation from X-ray scattering and secondary X-ray production is also a problem in this type of X-ray procedure since data imparted to the film or screen by X rays which do not travel directly from a fixed point in the X-ray tube to the film or screen is spurious data as far as the image is concerned. To reduce image degradation from this cause, it is a common practice to dispose an antiscatter grid, commonly referred to as a Bucky grid, between the subject and the film or screen.
Such Bucky grids are essentially multiply apertured collimators of the kind discussed above. The Bucky grid contains an array of small passages which transmit radiation that travels towards the film or screen along direct lines radiating from the origin point in the X-ray tube while the solid material of the grid absorbs X rays which arrive from other directions. Certain of the limitations of prior collimator constructions and construction methods as discussed above are also applicable to Bucky grids. Using prior constructions, aperture size is often sufficiently large and aperture density is often sufficiently low that an image of the grid itself is apparent in the desired X-ray image. The superimposed grid image may obscure the desired image to a significant extent. Radiographic equipment and procedures are often complicated by measures designed to minimize this problem. For example, it is a common practice to oscillate the Bucky grid during the exposure by acoustically induced vibration for example, in order to obscure the outline of the grid in the image.
The foregoing discussion of prior collimators and collimator manufacturing procedures has, for purposes of example, been primarily directed to collimators for X-ray systems. Similar collimators are used and similar problems are encountered in systems which utilize other types of high frequency radiation. For example, radioactive sources emitting gamma radiation or other wavelengths are sometimes employed in radiographic systems or the like which require collimators of the general kind discussed above.